Tension force is the pulling force transmitted along a string, rope, cable, or other flexible connector. It acts along the connector's length, can only pull (never push), and for an ideal massless string over a frictionless pulley, the tension is the same magnitude everywhere in the string.
Tension is a contact force that a flexible connector (string, rope, cable, chain) exerts on whatever it's attached to. It always pulls along the direction of the connector, and it can never push. If you try to compress a string, it just goes slack and the tension drops to zero.
On the AP Physics C exam, you almost always work with the ideal string model. The string is massless and inextensible, so tension has the same magnitude at every point along it, even when it bends over an ideal (frictionless, massless) pulley. The pulley changes the direction of the tension without changing its magnitude. That assumption is what lets you connect two objects in a system, like the two blocks of an Atwood machine, with one shared tension and one shared acceleration. Once a pulley has mass and friction with the string (a rotational dynamics setup), that assumption breaks, and the tension on each side of the pulley is different. That difference in tensions is exactly what provides the torque that spins the pulley.
Tension lives in Topic 2.3 (Newton's Laws of Motion) and shows up again in Topic 3.1 (Work-Energy Theorem). In Unit 2, tension is one of the standard forces you draw on a free-body diagram and feed into Newton's second law, and it's a classic Newton's third law pair (the rope pulls on the block, the block pulls back on the rope with equal magnitude). Connected-object systems are where tension earns its keep. The string is the physical link that forces two objects to share an acceleration constraint, which is the whole trick behind Atwood machines and pulley problems.
In Unit 3, tension matters for work-energy reasoning. A motor pulling a block by a string does work on the block through the tension force, and you compute that work as the tension times the displacement along the string. Knowing when tension does work (block dragged across a table) versus when it does zero work (tension perpendicular to motion, like the string in uniform circular motion) is a skill the exam tests directly.
Keep studying AP Physics C: Mechanics Unit 2
Newton's Third Law (Unit 2)
Tension is the cleanest third-law example on the exam. When a rope pulls a block forward, the block pulls back on the rope with equal magnitude. For an ideal massless string, Newton's second law on the string itself (F = ma with m = 0) is why the tension is uniform along it.
Atwood Machine (Unit 2)
The Atwood machine is basically a tension problem in costume. Two masses hang from one string over a pulley, so they share one tension T and one acceleration magnitude. You write Newton's second law for each mass separately and solve the pair. If the pulley has mass, the two sides have different tensions, and that difference torques the pulley.
Work-Energy Theorem (Unit 3)
Tension does work equal to T times the displacement along the string's direction. On the 2022 exam, a motor pulled a block across a rough table via a string, and the work done by tension fed straight into the work-energy theorem alongside the negative work of friction.
Centripetal Force (Unit 2)
When an object swings in a circle on a string, tension supplies some or all of the centripetal force. Centripetal force isn't a new force on your free-body diagram. It's a job description, and tension is often the force doing that job. In uniform circular motion that tension does zero work because it's always perpendicular to the velocity.
Tension is a workhorse on both sections. In multiple choice, expect free-body diagram questions (which way does tension point?), system problems where you find T in terms of masses and g, and conceptual traps like whether tension does work in circular motion (it doesn't, because it's perpendicular to velocity). On FRQs, tension shows up constantly. The 2022 FRQ Q1 had a motor pulling a block across a rough table by a string over a pulley, requiring Newton's second law with tension and friction, then energy analysis. The 2022 FRQ Q3 and 2024 FRQ Q3 both used strings attached to rotating objects (a disk and a pivoted rod), where the tension creates a torque. Your jobs are consistent across all of these. Draw tension along the string pointing away from the object, write F = ma or τ = Iα including it, and recognize when tensions differ on opposite sides of a massive pulley.
Both are contact constraint forces whose magnitudes you solve for (neither has a formula like mg). The difference is direction and sign. Tension pulls along the connector and can only pull, while normal force pushes perpendicular to a surface and can only push. A string goes slack instead of pushing; a surface lets go instead of pulling. On the exam, checking whether T ≥ 0 or N ≥ 0 is how you decide if a string goes slack or an object leaves a surface.
Tension always pulls along the string, away from the object it's attached to, and a string can never push.
For an ideal massless string over a frictionless, massless pulley, the tension has the same magnitude everywhere in the string.
If the pulley has mass (rotational dynamics), the tensions on the two sides are different, and that difference provides the torque that spins the pulley.
Tension is a constraint force, so you solve for it from Newton's second law rather than plugging into a formula.
Tension does work when it has a component along the displacement, like a string dragging a block, but does zero work in uniform circular motion because it's perpendicular to velocity.
In connected systems like the Atwood machine, the string forces both objects to share the same acceleration magnitude, which is the key constraint equation.
Tension is the pulling force transmitted through a string, rope, or cable along its length. On the AP exam you usually model strings as massless and inextensible, so the tension is uniform along the string and connected objects share an acceleration.
No. T = mg only when the object is in equilibrium (zero acceleration). If the object accelerates downward, T < mg; if it accelerates upward, T > mg. Assuming T = mg in an Atwood machine is one of the most common errors on FRQs.
Only if the pulley is ideal (massless and frictionless). If the pulley has mass and rotates with the string, the tensions on the two sides must be different, because the net torque from those tensions is what gives the pulley its angular acceleration.
Centripetal force isn't a separate force; it's the net inward force required for circular motion. Tension is a real force that often plays that role, like a string keeping a ball moving in a circle. You never draw 'centripetal force' on a free-body diagram, but you do draw tension.
It depends on the geometry. When a string pulls a block along its direction of motion, like the motor-and-pulley setup on the 2022 FRQ, tension does positive work equal to T times the displacement along the string. In uniform circular motion, tension is perpendicular to velocity and does zero work.